Influence of microalloying additions on thickness of grain boundary carbides in ferrite–pearlite steels

Abstract

For a series of plain C and microalloyed steels at two levels of Mn, the growth of grain boundary carbides has been monitored after heating to 920°C and cooling at 40 and 150 K min^?1 through the austenite–ferrite/pearlite transformation down to room temperature. In pearlite free steels, on cooling to room temperature, all the C in solution in the ferrite is able to precipitate as carbides at the boundaries and the grain boundary carbide thickness is dependent on the number of nucleation sites for precipitation. Increasing the cooling rate increases the number of sites and reduces the carbide thickness. In ferrite–pearlite steels, the grain boundary carbides form the ‘tails’ to the pearlite colonies. The thickness of the grain boundary carbide is related to the pearlite reaction, since the temperature at which this occurs controls both the thickness of the carbide nuclei and the amount of C available for precipitating out on these tails. Increasing the cooling rate and Mn content causes a decrease in the transformation temperature and leads to finer carbides. The pearlite nose transformation temperature must be ≦600°C to produce fine (≦0·2 μm) carbides. The austenite grain size, which controls the pearlite colony size, is also very important in determining the thickness of carbides, since the finer the grain size, the greater the carbide density and,for a given amount of C available for precipitation, the finer the resulting carbides. Faster cooling or a higher Mn content refine the pearlite colony size leading to finer carbides. Compared with C–Mn–Al steels, Nb and Ti microalloying additions result in coarser carbides and higher carbide densities. The increased carbide density is due to the finer austenite grain size and the coarser carbides are due to the finer grain size raising the transformation temperature. The implications of these observations on impact behaviour are discussed.

MST/1858     

Influence of grain boundary carbide density on impact behaviour of C–Mn–Nb–Al steels

Abstract

The influence of grain boundary carbide density on impact behaviour has been examined for C–Mn–Nb–Al steels by (i) normalizing at increasing temperatures above the Ac₃, and (ii) tempering for long times below the AC₁. Low normalizing temperatures (i) resulted in a large number of grain boundary carbides possibly because incomplete homogenization on austenitizing produces a high concentration of carbon at the boundaries. Raising the normalizing temperature reduced the number of grain boundary carbides as well as refining their size, but the expected improvement in impact behaviour was not realized because grain size also increased. Tempering at 680°C raised the grain boundary carbide density considerably and completely destroyed the pearlite colonies; tempering at 600°C (ii) gave a lower increase in carbide density and destroyed fewer pearlite colonies. Only small changes in grain size and grain boundary carbide thickness were noted so that the deterioration in impact behaviour obtained on tempering at 680°C could be ascribed mainly to this increase in grain boundary carbide density. Analysis of all the results suggests that an increase in grain boundary carbide density by 20 mm^?1 at constant grain size results in an increase in transition temperature of ~30 K. This agrees with the 20–30 K rise in transition temperature reported in a previously published paper which relied on a linear regression approach.

MST/424     

Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel

Abstract

The influence of prior austenite grain size d_γ, and true interlamellar spacing of pearlite S_t on the strength and fracture toughness of a eutectoid rail steel has been investigated. Specimens were machined from rail sections and heat treated to produce a wide variation in d_γ and s_t. Mechanical properties studied included 0·2% proof stress σ_0·2, ultimate tensile strength σ_u, tensile ductility δ, cleavage fracture stress σ_f, and plane strain fracture toughness K_1c. All tests were performed at a temperature of ?80°C. The values of σ_0·2 and σ_u increase as s_t decreases. The proof stress is related to the mean free distance λin the pearlitic ferrite by a Hall–Petch equation. A microstructural dependence similar to that of σ_0·2 is shown by σ_f and for all but the finest pearlites σ_f is interpreted as a shear stress controlled cleavage nucleation stress. The value of K_1c first decreases with decreasing s_t and then increases for the finest spacings. This behaviour is attributed to a change in the micromechanism of cleavage nucleation as the pearlite spacing changes from coarse to fine. The value of d_y has very little effect on K_1c, but δ decreases progressively as d_γ increases. The effect of d_γ on K_1c is negligible because the fracture process zone is much smaller than the grain size and therefore the grain boundaries cannot influence the fracture processes occurring at the crack tip. The tensile ductility is interpreted as the strain necessary to develop an internal microcrack which then propagates as a quasibrittle fracture. The size of the microcrack is shown to be related to the pearlite nodule size which in turn is related to d_γ.

MST/396     

Influence of intercritical annealing on strength and impact behaviour of niobium containing steels

Abstract

The influence of inter critical annealing at 730°C on the impact properties and strength of C–Mn–Al–Nb steels has been examined. For low Mn (0·56%), Nb steels, intercritical annealing resulted in improved impact performance and the impact transition temperature (ITT) was reduced by as much as 35 K with no change in strength. The improvement in impact performance is considered to be due to Mn segregating to the α/γ boundaries leading to refinement of the grain boundary carbides. This refinement increased with holding time at 730°C in accordance with an increased grain boundary segregation of Mn. Strength was not influenced because grain size remained unchanged on intercritical annealing. The improvement in impact behaviour was greater the longer the holding time at 730°C but was significant even after 15 min. Improvements occurred both on cooling from the austenitising temperature (9·20°C) to 730°C and on heating from room temperature to 730°C, the latter heat treatment being the more beneficial. For higher Mn (1·4%), Nb steels, improvements in impact performance resulting from intercritical annealing depended on cooling rate. Again, the Mn build-up in the y increases with time of intercritical annealing. Owing to the initial overall higher Mn level and finer grain size, the steels were susceptible to martensite formation if the cooling rate was too high. At a cooling rate of 40 K min - 1, improvements in impact behaviour occurred only after short intercritical annealing times (30 min) when only a small amount of martensite had formed. Long times caused a serious deterioration in impact behaviour due to the presence of high volume fractions of martensite. Slow cooling (1 K min^?1), however, ensured ferrite–pearlite structures and significant improvements in impact behaviour (20–60 K reductions in ITT) were noted on intercritical annealing with no change in strength. The short holding times required to achieve an improvement in impact behaviour in these fine grained steels are encouraging for the possible commercial exploitation of this heat treatment.

MST/1382     

Processing of steel for ultrafine ferrite grain structures

Abstract

The attainment of ultrafine ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very fine grain sizes. This paper presents a summary of research aimed at producing ultrafine ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain refinement (TGR), in which extrafine austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of <1 µm in a surface layer, and 1.5 µm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1.5 µm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previously refined by TGR to an intermediate grain size was shown to produce an ultrafine grain microstructure (<1 µm grain size) throughout the section of 1 mm thick strip. The hardness of ultrafine ferrite was shown to obey a linear relationship with the inverse square root of grain size, but with a lower slope than expected from the Petch relationship for yield strength.     

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract

Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains d_α formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size d_γ. In the cooling rate range studied (q= 0·05–5 K s^?1), d_α was found to be proportional to q^?0·26d_γ^0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q^?0·17d_γ^0·33. This was in close agreement with the dependence obtained in the present experiment.

MST/466     

Effect of pearlite morphology on impact toughness of eutectoid steel containing vanadium

Abstract

The effect of a change in the morphology of the pearlite colonies on the Charpy impact energy of a fully pearlitic steel containing 0·76%C, 1·20%Mn, and 0·085% V was examined over the range of testing temperatures from ?50 to 200°C. The change from a multicolony nodular pearlite structure produced from austenite of grain size 185 μm to a structure composed of individually formed colonies produced from austenite of grain size 25 μm caused a decrease in the transition temperature of 75 K and an almost 100% increase in the Charpy impact energy measured at room temperature. It is proposed that the impact toughness of pearlitic steel can be affected by pearlite morphology, at constant interlamellar spacing, only at temperatures above the ductile–brittle transition temperature of the ferrite, when local plastic deformation in the pearlitic ferrite at high angle boundaries can arrest propagating brittle cracks.

MST/730     

Influence of small vanadium and cobalt additions on microstructure and properties of ductile iron

Abstract

Ductile irons containing additions of up to 0·44% Vand 0·48%Co have been prepared and tested in the as cast state and after aferritising heat treatment. In the heat treated condition the microstructure of the alloyed irons consists of graphite nodules in aferrite matrix that contains afine dispersion (20–80 nm in dia.) of V₄C₃ particles. These carbide particles produce dispersion strengthening and refine the grain size of the ferrite, resulting in an iron of intermediate strength and high ductility.

MST/3079     

Modeling the dependence of strength on grain sizes in nanocrystalline materials

Abstract

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall–Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall–Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ^?1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.     

Effect of grain size and carbide thickness on impact transition temperature of low carbon structural steels

Abstract

A model previously developed by Petch enables an estimate of the impact transition temperature (ITT) of low carbon structural steels to be made from measurements of grain size and carbide thickness. In the present work, various laboratory heat treated and commercially processed structural steels were assessed in order to ascertain the validity of the Petch model. Laboratory heat treated steels, with carbon contents of <0.1 wt-%, with a range of grain and carbide sizes, showed that accurate predictions of ITT could be made provided that the original Petch model was modified. The revised model incorporated a measure of the largest grains in the material rather than an average grain size, and necessitated a change to the expression used by Petch for the variation in yield stress with temperature at high strain rates appropriate to a Charpy test. The present work also showed that cleavage is associated with the largest carbides in the steel. The modified model also successfully estimated, to within ±10 K, the ITT of some commercially processed structural steels, of similar carbon content to the laboratory treated steels, but containing much thicker carbides. The revised model was slightly less successful in predicting the impact behaviour of a laboratory processed structural steel having a slightly higher carbon content (0.16 wt-%). It is believed that this is primarily because pearlite plays a more significant role in the cleavage fracture process in this steel compared to the lower carbon steels, but its role is ignored in the model.     

Strengths of grain boundaries in pure metals

Abstract

The relative surface energies for brittle fracture along grain boundaries or along crystal planes, at low temperatures, are estimated and used in a criterion for the relative strengths of boundaries and cleavages. It is concluded that the boundary is weaker than the crystal in a wide range of metals; that it becomes weaker as the ratio of shear modulus to bulk modulus increases; and that brittle pure metals, such as iridium and molybdenum, fracture preferentially on grain boundaries. The critical modulus ratio is in all cases lower than that for the ductile–brittle cleavage transition.

MST/1154     

Metallography of bainitic transformation in austempered ductile iron

Abstract

An unalloyed nodular cast iron has been used to investigate the development of microstructure on heat treating in the bainite temperature region. Specimens were austenitised at 900°C for 1·5 h, then austempered for 1, 2, or 3 h at 250,300, and 350°C, respectively, and examined by light, transmission electron, and scanning electron microscopy. Experimental results indicate a microstructure consisting of a stable, highly enriched, retained austenite with one of two lower bainitic ferrite morphologies. One of these morphologies is carbide free acicular ferrite for specimens austempered at 350°C for 1 h and the other is bainitic ferrite in which carbide is distributed within the ferrite produced by different heat treatment conditions. Austempering at 350°C for 2 h and at 300°C for 1 and 2 h resulted in the formation of transition carbides in bainitic ferrite platelets. The η carbide was formed at 350°C for 2 h by precipitation from a bainitic ferrite supersaturated with carbon. By contrast, ? carbide was associated with austempering at 300°C for 1 and 2 h and precipitates either on the austenite twin/bainitic ferrite boundaries or within the bainitic ferrite. The fracture mode of tensile and impact specimens in the austempered condition was fully ductile compared with as cast specimens, which had mixed fracture characteristics.

MST/1646     

Role of silicon in influencing strength and impact behaviour of ferrite and its likely influence at ultrafine grain size

Abstract

Plates of fine grained Nb containing steels having Si contents of 0.03, 0.1, and 0.51% were austenitised at 920°C and cooled at 40 and 7.5 K min^-1 through the γ to αtransformation temperature range. Increasing the Si level from 0.03 to 0.1%resulted in a decrease in the yield strength and an improvement in impact performance for both cooling rates. Raising the Si level further to 0.51%Si caused the strength to increase and the impact behaviour to deteriorate. The results are explained in terms of Si segregating to the ferrite boundaries, increasing the activity of C and N, and thereby displacing these atoms from the boundary regions. In this way, although Si increases σ₀in the Hall–Petch relationship the K _y value is reduced. Consequently, at fine grain size, Si additions can actually reduce the strength of steels and, at ultrafine grain sizes (1 µm), this would be expected to result in very substantial softening.     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Effects of ferrite grain size and martensite volume fraction on dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels

Abstract

Effects of ferrite grain size and martensite volume fraction on quasistatic and dynamic deformation behaviour of 0·15C–2·0Mn–0·2Si dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens that had different ferrite grain sizes and martensite volume fractions, using a torsional Kolsky bar, and then the test data were compared in terms of microstructures, tensile properties, fracture mode, and adiabatic shear band formation. Under dynamic torsional loading, maximum shear stress and fracture shear strain increased with decreasing ferrite grain size and increasing martensite volume fraction. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5 to 15 μm in width were formed along the shear stress direction, and that voids or microcracks initiated at ferrites or martensite/ferrite interfaces below the shear band. The width of the shear band decreased as the ferrite grain size increased or the martensite volume fraction decreased. These phenomena were then analysed by introducing concepts of theoretical critical shear strain.     

Influence of grain size on hot ductility of plain C–Mn steels

Abstract

The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar₃ temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the hot ductility of the 0·65wt-%C steel. Intergranular tensile fracture at coarse grain size was by grain boundary sliding in the austenite resulting in a very wide ductility trough. Refining the grain size prevented intergranular failure occurring in the γ down to the lowest temperature examined: 700°C. Although the main influence of grain size is in controlling the width of the trough, the depth also increased with an increase in grain size.

MST/420     

Micromechanism of cleavage fracture in fully pearlitic steels

Abstract

The micromechanism of cleavage fracture in a fully pearlitic steel has been investigated. Uniaxial tensile and compression test specimens, together with single notched bend (SNB) and double notched bend (DNB) specimens, were heat treated such that the prior austenite grain size remained constant while the pearlite interlamellar spacing was varied. The SNB specimens were used to determine the cleavage fracture stress σ_fM, over the temperature range ?25 to ?196°C. The DNB specimens were used to study the initial stages of crack nucleation. The results indicate that pearlite can exhibit two different cleavage mechanisms which are dependent on the strength of the steel. For cleavage fracture stresses below about 2100 MN m^?2, fracture is nucleation controlled and involves shear linking of carbide nucleated microcracks before unstable cleavage can occur. Under these conditions, the cleavage fracture stress is dependent on temperature and is proportional to the uniaxial proof stress. For cleavage fracture stresses above 2100 MN m^?2, cracked carbides act directly as cleavage nuclei. Fracture is then propagation controlled and the cleavage fracture stress is independent of temperature. The transition from nucleation–controlled to propagation–controlled cleavage may be achieved by either a reduction in pearlite interlamellar spacing or a reduction in testing temperature.

MST/355     

Fracture behaviour of C–Mn steel multipass MMA weld metals at −60°C in Charpy V testing

Abstract

By observation of the fracture surfaces and of appropriate metallographic sections of C–Mn steel multipass MMA (manual metal arc) weld metals and simulated weld specimens which were fractured at ?60°C in Charpy V tests, it was found that the impact toughness of the specimen could be correlated with the length of the fibrous crack which was limited by unstable propagation of the cleavage crack; the latter could be initiated at a type of second phase particle transformed from carbon rich regions or non-metallic inclusions. The weakest zone in which the cleavage crack initiated was characterised by coarse grains of ferrite and the critical event which gave rise to unstable propagation of a cleavage crack was a crack in the ferrite grain larger than 30 μm cutting through the boundary and extending over the specimen. On the basis of these results, a model of the fracture mechanism is proposed and the effect of Mn content on increasing toughness is explained using the model.

MST/647     

Influence of carbon on hot ductility of steels

Abstract

Hot ductility, measured by reduction in area, has been determined over the temperature range 550–950°C for a series of plain C–Mn steels having the same base composition except for the carbon content, which was in the range 0·04–0·65 wt-%. A ductility trough was obtained for all the steels and minimum ductility values were similar. Raising the carbon content from 0·04 to 0·28 wt–% caused the ductility trough to move to lower temperatures and this was in agreement with the observed changes in transformation temperature. Tensile fracture at the minimum ductility temperature was along thin films of ferrite which formed round the austenite grains – generally by deformation–induced transformation. The softer ferrite allowed strain concentration to cause ductile voiding at the MnS inclusions, and the voids eventually linked up to give intergranular failure. Raising the carbon content above the 0·28% level caused a change in the fracture mode. Instead of the ductility troughs moving to lower temperatures, a shift of over 100 K to higher temperatures was observed. Intergranular failure now occurred in the austenite as a result of grain boundary sliding. It is suggested that this change in fracture mode is caused by carbon increasing the activation energy, and hence the critical strain required for dynamic recrystallization, so favouring the linking of cracks formed by grain boundary sliding.

MST/366